A study item called ‘Uplink transmit diversity for HSPA’, see RP-090987, has recently been completed in 3GPP. The objective of the study item was to perform the feasibility study to evaluate the system gain of uplink transmit diversity schemes for High Speed Packet Access (HSPA) as well as analyze the impact on the User equipment (UE) and base station implementation and complexity in a cellular radio system.
The baseline and typical UE implementation comprises a single uplink transmit antenna used for all types of uplink transmission. However, high end UEs may have and use multiple uplink transmit antennas for uplink transmission. This is commonly referred to as uplink transmit diversity. The objective of the transmit diversity transmission is to achieve higher uplink data rate and/or lower UE transmission power by the virtue of spatial, angular and/or temporal diversities.
A commonly used uplink transmit diversity consists of two uplink transmit antennas. The signals from two or more uplink transmit diversity antennas may be transmitted in different manner in terms of their phases, amplitude, power etc. This gives rise to different uplink transmit diversity schemes. Some well known schemes are:                Transmit beamforming open loop        Transmit beamforming closed loop        Switched antenna uplink transmit diversity open loop        Switched antenna uplink transmit diversity closed loop        Space time transmit diversity        
Typically, in any transmit diversity scheme, a set of parameters related to uplink transmit diversity are regularly adjusted by the UE. The objective is to ensure that the uplink transmission incorporates the desired spatial, temporal or angular diversities. This in turns improves uplink coverage, reduces interference, increases uplink bit rate and enables UE to lower its transmitted power.
The transmit diversity parameters may comprise of one or many of: relative phase, relative amplitude, relative power, relative frequency, timing, absolute or total power of signals transmitted on transmit diversity branches, etc.
The adjustment of all or a sub-set of these parameters is fundamental to transmit beamforming scheme. The objective of beamforming is to direct the uplink transmission or beam towards the desired base station, which is generally the serving base station. This allows the serving base station to decode the received signal more easily. Furthermore, high directivity of the beam towards the desired base station reduces the interference towards the neighboring base stations.
Similarly in case of switched antenna transmit diversity, transmit diversity parameter implies the selection of the most appropriate transmit antenna (e.g. in terms of radio condition) out of the available transmit diversity branches. By the virtue of using the most appropriate antenna for the uplink transmission, the UE can either reduce its power while retaining a given uplink information rate, or increase the information rate while retaining a given output power.
In open loop transmit diversity schemes, the UE autonomously adjusts the uplink transmit diversity parameters without the use of any network transmitted control signaling or commands. These schemes are simpler, although they may not show substantial gain in all scenarios.
On the other hand in closed loop transmit diversity schemes, the UE adjusts the uplink transmit diversity parameters by making use of a suitable network transmitted control signaling or commands. These commands or control signals reflect the uplink quality e.g. the quality measured at the base station. These commands (control signals) are signaled to the UE over the downlink. Furthermore, the commands can be sent exclusively to the UE to enable it to adjust the uplink transmit diversity parameters. Alternatively the UE can utilize any existing commands or signaling, which are originally intended for other purposes, to derive the uplink transmit diversity parameters. Examples of such implicit signaling or commands are transmit power control (TPC) commands and Hybrid Automatic Repeat Request (HARQ) Acknowledgement/Negative Acknowledgement (ACK/NACK) etc which are sent to the UE by the base station for uplink power control and uplink HARQ retransmission scheme respectively. The closed loop schemes have a potential of leading to a better performance gain due to the use of network controlled signaling for adjusting the uplink transmit diversity parameters.
Furthermore transmit diversity schemes can be used in any cellular radio system technology including Long Term Evolution (LTE), Wideband Code Division Multiple Access (WCDMA) or Global System for Mobile Communication (GSM). For instance in LTE, the switched antenna uplink transmit diversity is standardized in LTE release 8.
Reliability is particularly important in transmissions where an entire decision relies upon one or more simple commands such as on/off signaling, ACK and NACK responses, up and down power control commands, etc. Unreliable commands may cause actions to occur in reverse direction and in some cases may lead to unstable behavior.
The unreliability generally occurs due to bad radio conditions, low transmitted power level, poor coverage, high system load etc whereby the receiver cannot properly interpret the correct meaning of the command.
To ensure reliable operation, four sets of functionalities can be specified:                Reliability check        Behavior or action of UE and/or base station        Indication or reporting of unreliability events to network        Prevention of unreliability        
The reliability check can be based either on some signal strength or quality such as signal to interference ratio (SIR). It can also be based on some bit error rate (BER) target value. This means a received command is regarded as unreliable in case the received signal quality or strength falls between the thresholds or if the BER is higher than the target.
The behavior of UE or base station in response to unreliable command detection depends upon a particular functionality governed by the on/off command.
A number of uplink transmit diversity schemes exists which use network control signaling or commands to derive the uplink transmit diversity parameters. For instance certain schemes such as uplink transmit beamforming could make use of TPC commands or HARQ ACK/NACK sent by the base station to derive the uplink transmit diversity parameters. Similarly the switched antenna uplink transmit diversity scheme could make use of TPC commands or HARQ ACK/NACK for the antenna selection. Both these schemes have been studied within the scope of the recently completed study item in 3GPP, see RP-090987.
The above signaling (TPC or HARQ ACK/NACK or the like), which are transmitted on the downlink, are used to characterize the uplink quality. However due to poor downlink quality the commands or network signaling such as TPC can be erroneously received by the UE. For instance due to error the TPC, which is either 0 (e.g. DOWN) or 1 (e.g. UP), if transmitted as 1 can be interpreted as 0 or vice versa. This will lead to erroneous adjustment of transmit diversity parameters in case TPC (or similar commands or signaling) is used for this purpose. The erroneous adjustment of parameters may lead to incorrect beamforming or antenna switching. For instance the beam may be directed to the neighbor base station instead of the serving one. The uplink performance (i.e. UE with erroneous parameter setting due to incorrect received command) will deteriorate and the neighboring cells will receive excessive and unnecessary interference.
Further, US patent application No. 2008/123768 describes a system where TPC and HARQ indicator (ACK/NACK) for setting weighting of signals between the 2 transmit antennas. The HARQ quality indicator is not used when NACK is received.
The downlink quality varies with the radio conditions and particularly deteriorates for UEs which are close to the cell border or when the downlink load is high. Especially when being close to the cell border the UE is generally closer to the neighboring base stations and far from the serving one. Hence, the erroneously received commands (control signals), which cause incorrect setting of the uplink diversity parameters for the uplink transmit diversity transmission, will lead to even more substantial interference and lower performance.
It is therefore important to develop methods and arrangements, which can ensure that the values of the uplink transmit diversity parameters are correctly derived.